1. Field of the Invention
The present invention relates to an optical glass containing as glass-forming components both fluoride and phosphate ions, and not containing beryllium, arsenic, thorium, lead and other metal ions detrimental to health. This glass can be produced on an industrial scale and possesses a very high Abbe's number.
2. Description of the Prior Art
An optical glasses containing a fluoride-phosphate system, those which contain both alkali metaphosphates and fluorides in large amounts, as disclosed in Japanese Patent Publication 8,459/57, and those which are free from alkali metal metaphosphates and alkaline earth metal metaphosphates, as disclosed in Japanese Patent Publication No. 32,177/69 and so on, are hitherto reported. However, the optical glass of this kind gives rise to striae and devitrification resulting from vaporization of fluorine during fusing and casting and as a result the production of a large-size glass block on an industrial scale is difficult.